The present invention relates to the field of multijunction solar cells incorporating a plurality of heterogeneous layers of epitaxial material on semiconductor substrates. More particularly, the present invention relates to an improved method of patterning epitaxial layers using a single step wet etch process.
Solar cells are important renewable energy sources that have become widely deployed for both space and terrestrial applications. Today, the highest efficiency solar cells are realized using the multijunction solar cell approach. Multijunction solar cells typically consist of two, three, or more junctions, i.e., subcells, that are serially connected in a stack, as illustrated in FIG. 1A. The junctions are typically realized by growing a plurality of heteroepitaxial layers 9 on a semiconductor substrate 12. Each junction 114, 115, and 116 is designed to absorb from a separate portion of the solar energy spectrum, allowing for solar energy conversion with high efficiency. The junctions are separated by interconnection regions 117 and 118. The epitaxial layers 9 that make up the multijunction solar cells are chosen from a variety of semiconductor materials with different optical and electrical properties in order to absorb different portions of the solar energy spectrum.
Fabrication of multijunction solar cells is conventionally carried out on the wafer scale using conventional semiconductor processing methods that are known to those skilled in the art. A summary of processing steps for making a typical multi junction solar cell is found in Danzilio et al., cited below.
An important step in the fabrication of multijunction solar cells, illustrated in FIG. 1B, is the isolation of individual cells 100 on a semiconductor substrate 12, herein referred to as “mesa isolation.” Mesa isolation is done to eliminate electrical contact between adjacent solar cells on the same wafer, which then allows the mechanical separation of the individual cells 100 (also referred to as singulation) to take place without disturbing the edges of each individual cell.
In the mesa isolation step, the epitaxial layers 9 must be removed on all four edges of the individual solar cell chips, as can be seen in FIG. 1A and FIG. 2. If the bottom-most junction in the stack uses the substrate 12 as seen in FIG. 2 as the base region of the junction (e.g., germanium), then mesa isolation must extend into the substrate region, resulting in partially etched substrate 12, such that the etch depth into the substrate exceeds the minority carrier diffusion length in that base region.
Mesa isolation of cells can be achieved by using a number of techniques including, but not limited to, dry etching, partial (or full) cut using techniques such as saw or laser dicing, and wet etching. These techniques, as used in the prior art, can be reviewed briefly as follows:
Dry Etching: Dry etching is the removal of semiconductor material by exposing the material to plasma of reactive gases in vacuum chambers. Dry etching is a well-established processing technique in the semiconductor industry. However, it has found limited use in solar cell manufacturing due to low throughput and high costs involved in equipment setup and maintenance. Consequently, dry etching is not typically used for mesa isolation of multijunction solar cells.
Partial Cut: The partial cut technique uses a dicing saw or laser beam to cut partially through the wafer to isolate solar cells electrically. Mechanical removal of semiconductor material results in chipping and damage on sidewalls, which leads to poor electrical performance of the chip. It is difficult to control the actively absorbing area of the solar cell, resulting in a variation in performance. Furthermore, partial cut is also a low throughput technique. Nevertheless the partial cut technique is relatively cost-effective and is sometimes used.
Wet Etching: Wet etching is the removal of semiconductor material using chemicals in liquid phase. Wet etching is the preferred method found in the prior art, because it is cost effective, does not require sophisticated processing equipment, and wafer throughput is high. The present invention is a wet etching technique for multijunction solar cell processing.
Multijunction solar cells are formed of multiple epitaxial layers with different electrical, optical, and chemical properties. Semiconductor materials used in multijunction solar cells include, but are not limited to, indium gallium phosphide, indium phosphide, gallium arsenide, aluminum gallium arsenide, indium gallium arsenide, dilute nitride compounds, and germanium. For ternary and quaternary compound semiconductors, a wide range of alloy ratios can be used.
There are several wet etch chemistries commonly used in compound semiconductor manufacturing. A comprehensive list of wet etchants, along with etch rates and selectivity relationships was published by Clawson, Materials Science and Engineering, 31 (2001) 1-438. Typically, the wet etchants used for etching one class of semiconductor material are selective and will not etch certain other classes of semiconductor material. The selectivity of a wet etchant may also depend on alloy concentration of the compounds. Consequently, etching a full stack of epitaxial layers in multijunction solar cells for mesa isolation typically has required application of multiple wet etch chemistries.
Using multiple applications of selective (or nonselective) etchants typically results in jagged, non-smooth, and/or irregular mesa sidewall profile. FIG. 4 depicts an example of a multijunction solar cell found in the prior art, as would be produced by an image of a scanning electron microscope. In the fabrication of the solar cell shown in FIG. 4, different etch chemistries were used to etch each of the junctions 114, 115, and 116. Due to differences in chemistries and selectivity relationships, each of the three junctions and the substrate 112 has a distinctively different etch profile, resulting in the jagged, non-smooth, and/or irregular shape shown in FIG. 4. The shape of mesa sidewalls will vary depending on the semiconductor materials used in the solar cell and the chemicals used to etch them. Typically, such jagged mesa sidewall profiles result in larger sidewall surface areas (for a given mesa size and etch depth) compared to a uniform sidewall profile. Such larger sidewall surface areas may result in a higher leakage current on the perimeter of the solar cell, which in turn may result in reduced open circuit voltage and efficiency in multijunction solar cells.
In addition to perimeter leakage current, the use of multiple etchants has other disadvantages compared to single-etch chemistries including, for example:
1. Longer processing time.
2. Increased difficulty in controlling the etch rate and relative undercut of layers.
3. More chemical, water, and energy consumption during fabrication.
4. More chemical waste generation.
5. Multiple photolithography steps may be required to prevent damage to the sidewalls.
6. Uneven etching of different semiconductor layers.
Conventional processes have not been adequate to fabricate a cost-effective, high performance multijunction solar cell. Consequently there have been attempts to find nonselective etchants. The prior attempts are briefly described.
Turala et al. (CPV-7 Las Vegas, Nev. 2011) disclose a method using a high-viscosity bromine solution to etch III-V materials used in multijunction solar cells on germanium substrates. The solution proposed by Turala et al., however, requires the use of a silicon nitride hard mask, which must be deposited using plasma enhanced chemical vapor deposition. Using dielectric hard mask is not the preferred method in fabrication of solar cells because the use of photoresist masking, common in all semiconductor manufacturing, is much more cost-effective.
In a different approach, Zaknoune et al., J. Vac. Sci. Technol. B 16, 223 (1998), report an etching procedure that is nonselective for gallium arsenide and aluminum gallium indium phosphide, wherein the aluminum gallium indium phosphide quaternary compound has 35% aluminum phosphide, 15% gallium phosphide, and 50% indium phosphide. The etching procedure described by Zaknoune et al. uses a diluted solution of hydrochloric acid, iodic acid, and water to etch 300 nm of the quaternary compound grown on gallium arsenide substrate using a photoresist mask. The main application areas described in the paper by Zaknoune et al. are heterojunction bipolar transistors (HBT), various quantum well lasers (QWL), and high electron mobility transistors (HEMT) for which large conduction and valance band discontinuities are required. These devices are majority carrier devices wherein the large bandgap materials are typically used as barrier materials for majority carriers. Zaknoune et al. describes a system with one layer of epitaxy and does not recognize any sidewall problem related to multilayer epitaxy that is characteristic of solar cells.
The device requirements for multijunction solar cells are significantly different than for HBTs, QWLs, and HEMTs, largely because multijunction solar cells are minority carrier devices. Consequently the procedure described by Zahnoune et al. has no direct application to etching multijunction solar cell structures, which include a wide variety of semiconductor materials with a wide range of bandgaps (typically 0.67 eV to 2.25 eV).
The requirements for the mesa isolation step in the fabrication of multijunction solar cells can be summarized as follows:
1. Etchants that do not affect standard photoresist materials, such that conventional photolithography techniques can be used to define mesa patterns on semiconductor substrates and the exposed areas can be removed using wet etch chemistry. In particular, dielectric hard masks can thereby be avoided when defining mesa patterns.
2. A nonselective etch that etches all materials in the range of bandgaps from 0.67 eV (germanium) to 2.25 eV (gallium phosphide). FIG. 3 shows some of these semiconductor materials and their alloys in the range of bandgap energies.
3. Small sidewall surface area to result in reduced leakage current.
4. Ability to change and control the relative etch rates of epitaxial layers within the solar cell, for example through changing the composition of the etchant. An etchant that etches all materials in the bandgap range with similar etch rate may be preferred.
5. Using a single mixture of chemicals such that the wet etch for mesa isolation can be completed in a single step with minimal waste generation and water and energy consumption.
What is needed is a fabrication process better adapted to multijunction solar cell manufacturing.